The present invention relates to radiation beam therapy systems and more particularly to an improved gantry for use in such systems to support and rotate a radiation beam transport and delivery system around a stationary patient so as to precisely deliver a treatment beam to a predetermined target isocenter within the patient from several different angles during patient treatment. Preferably, the gantry is earthquake resistant, roller-supported for turning on its axis of rotation over a range equal to or greater than plus or minus 185 degrees from a home position and modular in construction for relatively inexpensive construction and assembly within existing structures or structure under construction.
U.S. patent application Ser. No. 07/163,611, filed Mar. 3, 1988. describes and illustrates a radiation beam therapy system including several different treatment stations each including a gantry for supporting and rotating a radiation beam transport and delivery system on an axis of rotation around a stationary patient to deliver a treatment beam to a predetermined target isocenter within the patient from several different angles during patient treatment. It is stated that the gantry structure may conform to the gantry described in a publication of the Harvard Cyclotron Laboratory entitled "Preliminary Design Study for a Corkscrew Gantry" authored by A. M. Koehler and an application for United States Patent entitled "Beam Redirecting" filed by Harvard University, Boston, Mass. as Ser. No. 076,868 on July 23, 1987, now U.S. Pat. No. 4,812,688, issued 3/14/89. The publication and patent application describe the gantry as including a single large ring structure carrying heavy counterweights and a portion of a beam delivery system. The ring is supported only by rollers for turning in a vertical plane on a horizontal axis. Such a design is laterally and axially unstable particulary in seismic environments. Because of such instability, even with very careful machining of the ring, the gantry is not capable of reliably meeting a one millimeter concentricity of isocenter specification for a treatment beam delivered by the beam delivery system. More recently, a different gantry design has been proposed by MEDCYC of Lansing, Mich. The MEDCYC design includes two concentric ring structures separated by axially extending brace members which support a heavy cyclotron and an opposing counterweight structure. A beam from the cyclotron is directed at an isocenter located on the axis of rotation of the pair of rings. Due to the weight of the cyclotron and counterweights, the brace members are subject to radial deflection with rotation of the gantry. Such deflection will materially impair the ability of the gantry to consistently meet the isocenter concentricity requirements for radiation beam therapy systems. Furthermore, the MEDCYC design will be subject to undesired instabilities during seismic conditions.
Accordingly, there is a need for a gantry which is able to maintain the desired concentricity of its isocenter during seismic conditions and which is relatively simple in its construction and assembly. The gantry of the present invention satisfies such needs.